As the fleet of commercial and military aircraft ages, certain time delayed problems are becoming more and more current. Among these problems, aircraft have been experiencing metal fatigue due to the cyclic cabin pressurization when climbing to cruising altitude and cabin depressurization when landing. The cyclic effect of landing and climbing to cruising altitude already has caused some aircraft structural failure and is predicted to become a major cause for the retirement of otherwise good aircraft in the coming years. The pressurization to approximately 12 pounds per square inch difference at altitude has in effect created a cold work of the aluminum alloy skin with microscopic crack propagation at certain points of the aircraft skin. The propagation of the cracks lead to metal fatigue and eventual catastrophic destruction of the skin.
Aircraft operating in the commercial or military environment must, by design, transport passengers or payload from one location to another. Particularly modern commercial jet aircraft, that typically takeoff from an airport and climb to a cruising altitude, require the cabin to be pressurized in order to maintain inside cabin pressure close to sea level pressure and thereby by assure the comfort of the passengers and crew. Because of the necessity of pressurization at higher altitudes, the pressure difference between inside the cabin and the outside ambient causes the metal skin of the aircraft to deflect, that is to bulge outward, in order to sustain the higher inside pressure.
The aluminum panels covering the fuselage section of the aircraft have sufficient strength to easily maintain structural integrity as long as the pressure is maintained constant. However, the aircraft must of necessity land to discharge the passengers and this necessitates the pressure differential between the inside of the fuselage and the ambient being reduced to zero. With a zero pressure difference, no metal deflection, or bulging is occurring. As the cycle repeats, a certain amount of internal molecular rearrangement in the aluminum panels forming the aircraft skin, occurs (similar to the cold working of metal) during each pressurization-depressurization cycle. This molecular rearrangement may displace some of the atomic crystals in the skins.
Normally, the minute amount of flexing occurring during cabin pressurization does not displace the crystal lattice of the metal significantly as long as the metal remains in an elastic deformation configuration with respect to metal stress. The pressurization-depressurization cycle continues over many tens of thousands of landings and takeoffs with corresponding cycles of the flexing of the skin. As in cold work of metals, at some point the metal becomes harder due to the rearrangement of atoms in the crystal lattice. This of course increases the resistance of the metal to yield to an applied force. When the cycles continue, small fissures start to show up near the displaced layers of atoms. These fissures allow the remaining metallic crystals to carry the load of the separated atoms, thereby increasing the stress. Stress is the product of force divided by area. The remaining atoms carrying the load now are under a greater stress. The atoms yield one by one, as would the constituents of a tug of war. The fissures are propagated into small cracks and subsequently larger cracks. At some point the remaining atoms holding the load are overwhelmed and a catastrophic failure occurs.
One solution to this problem is that the manufacturer could increase the thickness of the aluminum skin, which would give the outside wall additional strength and reduce the flexure, thereby preventing metal fatigue for close to an indefinite timespan. This solution would increase the cost and more importantly, increase the weight of the aircraft. The increased aircraft weight would, in turn, reduce the payload of the aircraft, and thereby significantly increase the per passenger mile operating cost. Moreover, this solution is not practical for the many thousands of aircraft already in service.
A better solution would be to find a technique and structure to strengthen the existing the aluminum skin of aircraft. Any practical solution to eliminate, or extend well into the future, the problem of metal fatigue by strengthening the existing aluminum skin of an aircraft must meet several requirements. First, the solution must be of reasonable cost and second, the solution must not significantly increase the weight of the aircraft. Further, any solution must not increase the fire hazard in the aircraft in the event of a crash, and particularly must not emit any noxious gases when burning. In addition, the solution should be of a type which can be used to upgrade existing aircraft, as well as being included in the manufacture of new aircraft.
One known material which could be useful to provide a solution to the metal fatigue problem is a polyisocyanurate foam material. This material has a low density, does not burn and adheres to the aluminum components of an aircraft. Further, polyisocyanurate foam material has a tensile and compressive strength sufficient to prevent the aluminum skin from fracturing and is non-soluble in any solvent typically found on an aircraft. In addition, polyisocyanurate foam is an excellent insulating material and does not absorb moisture to any significant extent.